1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus which lights a discharge lamp by a high frequency current generated by a switching element.
2. Prior Art
FIG. 34 is a circuit diagram showing a construction of a conventional discharge lamp lighting apparatus. In FIG. 34, a reference symbol IV denotes an inverter circuit which is connected to a direct current power supply E, and switches a direct current of the direct current power supply E so that the direct current is converted into a high frequency current, an LAC1 denotes a discharge lamp load circuit for lighting a discharge lamp LA by a high frequency current generated by the inverter circuit IV, and an NP1 denotes a protective circuit which detects a fault of the discharge lamp load circuit LAC1, and outputs a control signal for stopping an operation of the inverter circuit IV.
The following is a detailed description on each of the above circuits.
The inverter circuit IV includes a starting circuit, a pair of MOS-FETs Q1 and Q2 (hereinafter, referred to as switching element Q1 and Q2), an inverter control circuit IC1 (hereinafter, referred to as IV control circuit IC1), and a frequency control circuit FC1. More specifically, the starting circuit is constructed in a manner that a starting resistor R1 and a control power supply capacitor C1 are connected in series and a constant voltage diode DZ1 is connected in parallel with the control power supply capacitor C1. The pair of switching elements Q1 and Q2 are connected in series between both electrodes of the direct current power supply E. The inverter control circuit IC1 controls the switching elements Q1 and Q2. The frequency control circuit FC1 sets a switching frequency of the switching elements Q1 and Q2 via the IV control circuit IC1. The IV control circuit IC1 has terminals; more specifically, a power supply terminal 1 (hereinafter, referred to as terminal 1) is connected to the control power supply capacitor C1, output terminals 2, 3 and 4 (hereinafter, referred to as terminals 2, 3 and 4) are connected to the switching elements Q1 and Q2, and oscillation control terminals 6 and 7 (hereinafter, referred to as terminals 6 and 7) are connected to the frequency control circuit FC1. Moreover, the frequency control circuit FC1 is composed of a main oscillation resistor R2 and an oscillation capacitor C2 which are connected between a negative electrode of the direct current power supply E and the terminals 6 and 7 of the IV control circuit IC1, respectively. In this manner, the IV control circuit IC1 oscillates at a frequency f=K* (current flowing from the terminal 6 of the IV control circuit, which has a constant direct current potential) with respect to a constant K determined by a capacitance of the oscillation capacitor C2, and thereby, the switching elements Q1 and Q2 make a switching operation at the frequency f.
Next, the following is a description on the discharge lamp load circuit LAC1.
As shown in FIG. 34, the discharge lamp load circuit LAC1 is composed of a ballast chock T1, a discharge lamp LA having electrodes F1 and F2, and a coupling capacitor C4 which are connected in series between both terminals of the switching element Q2, and further of a starting capacitor C3 connected in parallel with the discharge lamp LA.
On the other hand, the protective circuit NP1 is so constructed that the protective circuit NP1 detects a peak-to-peak voltage (Vmaxxe2x88x92Vmin) of a waveform of high frequency voltage between the electrode F1 side terminal of the ballast chock T1 and a negative electrode of the direct current power supply E by detection capacitors C5 and C6 connected to the discharge lamp load circuit LAC1, diodes D1 and D2 and a capacitor C7. Then, when a direct current voltage generated in both terminals of the capacitor C7 exceeds a Zener voltage of a constant voltage diode DZ2, the protective circuit NP1 outputs a signal to an oscillation stop terminal 5 (hereinafter, referred to as terminal 5) of the IV control circuit IC1 connected to the protective circuit NP1 so that a switching operation of the switching elements Q1 and Q2 is stopped. In this case, when the discharge lamp LA is normally lighting, the direct current voltage of the capacitor C7 is set so as to become lower than a Zener voltage of the constant voltage diode DZ2. Therefor, the protective circuit NP1 is not operated. Moreover, a resistor R4 is used for discharging a charge stored in the capacitor C7 when a power supply is turned off, and a resistor R16 and a capacitor C11 divide and control a voltage inputted to the terminal 5, and smooth an external high frequency noise, to prevent a malfunction of the IV control circuit IC1.
Next, the following is a description on an operation of a conventional discharge lamp lighting apparatus.
The discharge lamp is started up, and then, when a current is supplied to the inverter circuit IV from the direct current power supply E, the control power supply capacitor C1 is charged by a starting current flowing through the starting resistor R1 from the direct current power supply E. When a voltage of the terminal 1 of the IV control circuit IC reaches a predetermined operating voltage, the IV control circuit IC1 oscillates at a frequency f determined by the frequency control circuit FC1 so that a high frequency signal is outputted to the switching elements Q1 and Q2 from its terminals 2 and 4. Then, the switching elements Q1 and Q2 are alternately turned on and off, and thereby, a high frequency current is supplied to the discharge lamp load circuit LAC1. By the high frequency current, a series circuit comprising the ballast chock T1 and the starting capacitor C3 (for the coupling capacitor C4 is designed so as to have a capacitance several times as much as that of the starting capacitor C3, the coupling capacitor C4 has no influence on the following resonance phenomenon) generates an LC resonance. Subsequently, a high voltage is generated in the starting capacitor C3, that is, between both terminals of the discharge lamp LA. Thus, the discharge lamp LA is started, and continues to light at a frequency f. In this case, the control power supply capacitor C1 is connected in parallel with the constant voltage diode DZ1, so that a voltage applied to the terminals 1 of the IV control circuit IC1 is limited by a Zener voltage of the constant voltage diode DZ1.
Next, the following is a description on an operation of a conventional protective circuit NP1.
When the discharge lamp LA is lighting, a high frequency voltage as shown in FIG. 35 is generated between the electrode F1 side terminal of the ballast chock T1 and a negative electrode of the direct current power supply E. The high frequency voltage is generated so as to be overlapped with a constant direct current voltage. In the protective circuit NP1, a peak-to-peak voltage (Vmaxxe2x80x94Vmin) is detected by the detection capacitors C5 and C6 and the diodes D1 and D2 which are connected between the ballast chock T1 and the direct current power supply E, and further, is converted into a direct current voltage by the capacitor C7, and thus, is inputted to the constant voltage diode DZ2. In this case, when the discharge lamp LA is normally lighting, the direct current voltage of the capacitor C7 is set so as to become less than a Zener voltage of the constant voltage diode DZ2; therefore, no oscillation stop signal is outputted to the IV control circuit IC1 from the protective circuit NP1.
However, for example, in the case where the discharge lamp LA is rectified and lighting in the end of its file, a high frequency lamp voltage of the discharge lamp LA rises up; for this reason, a voltage of the capacitor C7 becomes higher than the Zener voltage of the constant voltage diode DZ2. Whereupon the protective circuit NP1 outputs an oscillation stop signal to the terminal 5 of the IV control circuit IC1, and further, by the oscillation stop of the IV control circuit IC1, a switching operation of the switching elements Q1 and Q2 is also stopped. As a result, that prevents the switching elements Q1 and Q2 from being abnormally exothermic, and a temperature in the vicinity of the electrodes F1 and F2 of the discharge lamp LA from becoming abnormally high to break down the discharge lamp LA. In this case, the oscillation stop state of the IV control circuit IC1 is reset at the time when a voltage of the control power supply capacitor C1 becomes less than a predetermined voltage, and an oscillation is started at the time when a voltage of the control power supply capacitor C1 becomes more than the predetermined voltage.
Moreover, in the case where a high resonance voltage is generated in the starting capacitor C3, a large current flows through the ballast chock T1 and the starting capacitor C3. Therefore, in the case where the discharge lamp LA is not lighting because of being defective or in the end of life, a voltage between terminals of the starting capacitor C3 is continuously kept abnormally high, and a direct current voltage of the capacitor C7 becomes higher than a Zener voltage of the constant voltage diode DZ2. Thus, in the same manner as described above, the protective circuit NP1 outputs an oscillation stop signal to the terminal 5 so as to stop an oscillation of the inverter circuit IV. As a result, it is possible to prevent an excessive current from continuously flowing through the ballast chock T1 and the starting capacitor C3 and the ballast chock T1 and the starting capacitor C3 from being broken down.
Moreover, in the case where the discharge lamp LA is dismounted during lighting, a resonance current flows through a series circuit comprising the ballast chock T1 and the detection capacitors C5 and C6, and thereby, the direct current voltage of the capacitor C7 becomes higher than the Zener voltage of the constant voltage diode DZ2. For this reason, the protective circuit NP1 outputs an oscillation stop signal to the terminal 5 so as to stop an oscillation of the inverter circuit IV. In this manner, in the case where the discharge lamp LA is dismounted during lighting, the oscillation of the inverter circuit IV is stopped, and then, no high frequency current flows through the discharge lamp load circuit LAC1; therefore, no high frequency voltage is generated terminals in a socket of the discharge lamp LA. As a result, it is possible to prevent accidents such as a ground fault occurring in lamp replacement.
However, the above conventional discharge lamp lighting apparatus shown in FIG. 34 has the following problems. The discharge lamp lighting apparatus detects a voltage difference between the maximum value and the minimum value of a high frequency voltage waveform between the electrode F1 side terminal of the ballast chock T1 and the negative electrode of the direct current power supply E. Then, by taking advantage of the fact that the above voltage difference becomes higher in abnormal cases (rectification lighting, no-lighting, no-load) than that in the case where the discharge lamp LA is normally lighting, an oscillation of the inverter circuit IV is stopped; for this reason, it is very difficult to make a circuit constant design for determining a protection level of the protective circuit NP1. Namely, in order to enhance a reliability of the protective circuit NP1, a sufficient margin needs to be left so that the protective circuit NP1 does not output an oscillation stop signal during normal lighting of the discharge lamp LA, and on the other hand, a sufficient margin needs to be set so that the protective circuit NP1 securely outputs an oscillation stop signal during abnormal lighting of the discharge lamp LA. As is evident from the circuit diagram shown in FIG. 34, the voltage difference detected by the protective circuit NP1 is, after all, a voltage applied to the discharge lamp LA (i.e., both terminals of the starting capacitor C3). In general, considering that a lamp voltage of the discharge lamp LA greatly varies according to a different between individual products and an environmental temperature, there is a problem, in this fault detecting system of the conventional protective circuit NP1 that the aforesaid two design margins cannot be set sufficiently large. In particular, in the discharge lamp lighting apparatus having a dimming function, a lamp voltage greatly rises when a lamp current of the discharge lamp LA is lowered to reduce a lumen output. Therefore, as a design of the protective circuit NP1 is very difficult, there is a problem that the above protective circuit NP1 cannot be actually applied to the discharge lamp lighting apparatus having a dimming function.
The present invention has been made in order to solve the above problems. It is, therefore, a first object of the present invention to provide a discharge lamp lighting apparatus which can take a sufficient design margin of a protective circuit, and can make high a reliability of the protective circuit and readily make a design of the protective circuit by securely distinguishing a normal lighting state from an abnormal lighting state.
Further, a second object of the present invention is to provide a discharge lamp lighting apparatus which can detect various faults of discharge lamp lighting apparatus, such as rectification lighting, no-lighting, and a no-load state, and can securely control an operation of an inverter circuit.
Further, a third object of the present invention is to provide a discharge lamp lighting apparatus having a preheat function of an electrode of a discharge lamp, which can securely light the discharge lamp and can securely control an operation of an inverter circuit in a fault state.
Further, a fourth object of the present invention is to provide a discharge lamp lighting apparatus which can securely light a discharge lamp in the case where an operating point in a steady state of the discharge lamp approaches or passes a resonance frequency of a discharge lamp load circuit, and can securely control an operation of an inverter circuit in a fault state.
Further, a fifth object of the present invention is to provide a discharge lamp lighting apparatus which can securely restart a discharge lamp after power supply is reset even in the case of an instantaneous failure of power supply, and can securely control an operation of an inverter circuit in a fault state.
Further, a sixth object of the present invention is to provide a discharge lamp lighting apparatus having a dimming function of a discharge lamp, which can take a sufficient design margin of a protective circuit securely light the discharge lamp by securely distinguishing a normal lighting state from an abnormal lighting state, securely control an operation of an inverter circuit in a fault state and have protective circuit having a high reliability.
Further, a seventh object of the present invention is to provide a discharge lamp lighting apparatus which has a low electrode loss consumed in an electrode of a discharge lamp, and has a high energy efficiency.
In order to achieve the above objects, the present invention provides a discharge lamp lighting apparatus comprising: a direct current power supply; a switching element for switching a direct current supplied from the direct current power supply so as to generate a high frequency current; a discharge lamp load circuit which is constructed in a manner that a discharge lamp and a coupling capacitor are connected in series, and the discharge lamp is lit by a high frequency current generated by the switching element; a protective circuit which detects a voltage generated in the coupling capacitor, and output a control signal; and a switching element control circuit for controlling the switching element by the control signal outputted from the protective circuit.
Further, the present invention provides the discharge lamp lighting apparatus wherein the protective circuit is composed of: a voltage detecting unit for detecting a voltage generated in the coupling capacitor, and converting the detected voltage into a direct current voltage; a comparator unit for comparing the direct current voltage detected and converted by the voltage detecting unit with a reference voltage; and a control signal output unit for generating and outputting a control signal on the basis of the comparative result made by the comparator unit.
Further, the present invention provides the discharge lamp lighting apparatus wherein the voltage detecting unit includes a divided resistor and a constant voltage diode for dividing a voltage inputted to the voltage detecting unit from the coupling capacitor, and is constructed so as to output a voltage divided by the divided resistor and the constant voltage diode to the comparator unit.
Further, the present invention provides the discharge lamp lighting apparatus wherein the comparator unit has at least two different reference voltages, and is a window type comparator which is constructed so as to compare a direct current voltage outputted from the voltage detecting unit with the at least two reference voltages.
Further, the present invention provides the discharge lamp lighting apparatus wherein the direct current voltage outputted from the voltage detecting unit is compared with two different reference voltages by the comparator unit, and when the voltage becomes lower than a reference voltage on a low voltage side or becomes higher than a reference voltage on a high voltage side, the control signal output unit outputs a stop signal or an output reducing signal of the switching element to the switching element control circuit.
Further, the present invention provides the discharge lamp lighting apparatus wherein the reference voltage of the comparator unit is set so as to be variable.
Further, the present invention provides the discharge lamp lighting apparatus wherein a plurality of discharge lamp load circuits having a coupling capacitor and a discharge lamp are driven by a high frequency current outputted from the switching element, and said protective circuit is provided with voltage detecting units each for detecting a voltage generated in the coupling capacitor of each of the discharge lamp load circuits, and converting the detected voltage into a direct current voltage; comparator units each for comparing the direct current voltage detected and converted by the voltage detecting unit with a reference voltage; and a control signal output unit for collecting outputs from the comparator units provided for the plurality of discharge lamp load circuits so as to generate a single control signal, and outputting the single control signal to the switching element control circuit.
Further, the present invention provides the discharge lamp lighting apparatus wherein the protective circuit is provided with a mask circuit for masking a control signal outputted from the protective circuit for a predetermined time.
Further, the present invention provides the discharge lamp lighting apparatus wherein the apparatus further includes an over resonance detection circuit for detecting a high frequency current supplied to the discharge lamp load circuit and outputting a control signal to the switching element control circuit, so that the switching element is controlled by the control signal from the protective circuit and the control signal from the over resonance detection circuit via the switching element control circuit.
Further, the present invention provides the discharge lamp lighting apparatus wherein the apparatus further includes an over resonance detection circuit for detecting a high frequency current supplied to the discharge lamp load circuit and outputting a control signal to the switching element control circuit, so that when the high frequency current detected by the over resonance detection circuit reaches a predetermined current value, even during a masking time of the protective circuit, the over resonance detection circuit outputs a stop signal or an output reducing signal of the switching element to the switching element control circuit.
Further, the present invention provides the discharge lamp lighting apparatus wherein the apparatus further includes a service interruption in the case restoring circuit for automatically resetting the mask circuit when a feed from the direct current power supply is shut off, so that after the feed is restored, the mask circuit is operated so as to mask a control signal outputted from the protective circuit to the switching element control circuit for a predetermined time.
Moreover, the present invention provides a discharge lamp lighting apparatus comprising: a direct current power supply; a switching element for switching a direct current supplied from the direct current power supply so as to generate a high frequency current; a discharge lamp load circuit which is constructed in a manner that a discharge lamp and a coupling capacitor are connected in series, and the discharge lamp is lit by a high frequency current generated by the switching element; a switching element control circuit for controlling the switching element; and a plurality of starting capacitors which are connected in parallel with the discharge lamp, at least one of the starting capacitors being connected to the switching element side with respect to the discharge lamp.
Further, the present invention provides the discharge lamp lighting apparatus wherein a plurality of discharge lamp load circuits each having a coupling capacitor and a discharge lamp are driven by a high frequency current outputted from the switching element, and the protective circuit is provided with a first voltage detecting unit for detecting a stepped-up voltage of each coupling capacitor of the discharge lamp load circuit, and converting the detected voltage into a direct current voltage; a second voltage detecting unit for detecting a dropped voltage of each coupling capacitor, and converting the detected voltage into a direct current voltage; a first comparator unit for comparing the stepped-up direct current voltage detected by the first voltage detecting unit with a reference voltage; a second comparator unit for comparing the drop direct current voltage detected and converted by the second voltage detecting unit with a reference voltage; and a control signal output unit for generating a control signal on the basis of an output from any of the first or second comparator units, and outputting the single control signal to the switching element control circuit.
Further, the present invention provides the discharge lamp lighting apparatus wherein the first voltage detecting unit includes a divided resistor and a constant voltage diode for dividing a voltage of each coupling capacitor, and reverse current blocking diodes interposed between the divided resistor and each coupling capacitor, and outputs the voltage divided by the divided resistor and the constant voltage diode to the first comparator unit, and the second voltage detecting unit includes a divided resistor and a constant voltage diode for dividing a predetermined voltage, and reverse current blocking diodes interposed between the divided resistor and each coupling capacitor, and outputs the voltage divided by the divided resistor and the constant voltage diode to the second comparator unit in the case where any voltage of each coupling capacitor is higher than the predetermined voltage, and further, is constructed in a manner that in the case where any voltage of each coupling capacitor is lower than the predetermined voltage, the predetermined voltage is applied to a coupling capacitor having a lower voltage via the reverse current blocking diode.
Further, the present invention provides the discharge lamp lighting apparatus wherein one end of each reverse current blocking diode of the second voltage detecting unit is connected to a starting capacitor side of the discharge lamp.
Further, the present invention provides the discharge lamp lighting apparatus wherein the first voltage detecting unit includes divided resistors and constant voltage diodes each for dividing a voltage of each coupling capacitor, and reverse current blocking diodes interposed between the constant voltage diodes and the first comparator unit, and outputs the voltage divided by the divided resistor and the constant voltage to the first comparator unit via the diode reverse current blocking diodes, and the second voltage detecting unit includes a divided resistor and a constant voltage diode for dividing a predetermined voltage, and reverse current blocking diodes interposed between the constant voltage diode and each of the constant voltage diodes of the first voltage detecting unit, and outputs the voltage divided by the divided resistor and the constant voltage diode to the second comparator unit in the case where any voltage of each coupling capacitor is higher than the predetermined voltage, and further, is constructed in a manner that in the case where any voltage of each coupling capacitor is lower than the predetermined voltage, the predetermined voltage is applied to a coupling capacitor having a lower voltage via the reverse current blocking diode, the divided resistor of the first voltage detecting unit and the constant voltage diode.
As is evident from the above description, the present invention has the aforesaid construction; and therefore, has the following effects.
More specifically, the present invention provides a discharge lamp lighting apparatus comprising: a direct current power supply; a switching element for switching a direct current supplied from the direct current power supply so as to generate a high frequency current; a discharge lamp load circuit which is constructed in a manner that a discharge lamp and a coupling capacitor are connected in series, and the discharge lamp is lit by a high frequency current generated by the switching element; a switching element control circuit for controlling the switching element. Further, the discharge lamp lighting apparatus includes a protective circuit which detects a voltage generated in the coupling capacitor, and output a control signal. Therefore, it is possible to securely distinguish a normal lighting state from an abnormal lighting state, and to stably light the discharge lamp in the normal lighting state. Moreover, it is possible to obtain a discharge lamp lighting apparatus which can control an oscillation of an inverter circuit in a fault state by securely operating the protective circuit, and has a high reliability.
Further, the protective circuit is composed of: a voltage detecting unit for detecting a voltage generated in the coupling capacitor, and converting the detected voltage into a direct current voltage; a comparator unit for comparing the direct current voltage detected by the voltage detecting unit with a reference voltage; and a control signal output unit for generating and outputting a control signal on the basis of the comparative result made by the comparator unit. Moreover, the comparator unit has at least two different reference voltages, and is a window type comparator which is constructed so as to compare a direct current voltage outputted from the voltage detecting unit with at least two reference voltages. Therefore, it is possible to detect a fault not only in a rectification lighting 1 state that a detection voltage steps up as compared with the fully normal lighting state, but also in a rectification lighting 2 state that a detection voltage steps up as compared with the fully normal lighting state and in a no-lighting state, and thus, to detect various faults generated in the discharge lamp.
The voltage detecting unit includes a divided resistor and a constant voltage diode for dividing a voltage inputted to the voltage detecting unit from the coupling capacitor, and is constructed so as to output a voltage divided by the divided resistor and the constant voltage diode to the comparator unit. Therefore, it is possible to largely set a difference in a reference voltage between the normal lighting state and the abnormal state, and thus, to further improve a reliability of the protective circuit.
The direct current voltage outputted from the voltage detecting unit is compared with two different reference voltages by the comparator unit, and when the voltage becomes lower than a reference voltage on a low voltage side or becomes higher than a reference voltage on a high voltage side, the control signal output unit outputs a stop signal or output reducing signal of the switching element to the switching element control circuit. Therefore, it is possible to securely detect various faults generated in the discharge lamp, and by stopping or reducing an output to the discharge lamp, it is possible to prevent a breakdown or ground fault of the discharge lamp, the discharge lamp load circuit or the like.
The reference voltage of the comparator is set so as to be variable. Therefore, it is possible to more precisely set a reference value in accordance with a characteristic of the discharge lamp.
Each of the plurality of discharge lamp load circuits is provided with a voltage detecting unit for detecting a voltage generated in the coupling capacitor, and converting the detected voltage into a direct current voltage; a comparator unit for comparing the direct current voltage detected by the voltage detecting unit with a reference voltage; and a control signal output unit for collecting an output from the comparator units provided in the plurality of discharge lamp load circuits so as to generate a single control signal, and outputting the single control signal to the switching element control circuit. Therefore, it is possible to detect a fault at the point of time when any discharge lamps are in a fault state, and to reduce the number of components of the control signal output unit.
The protective circuit is provided with a mask circuit for masking a control signal outputted from the protective circuit for a predetermined time. Therefore, it is possible to obtain a discharge lamp lighting apparatus which can securely light a normal discharge lamp, and can securely stop an oscillation in a fault state. Moreover, the protective circuit is applicable to a discharge lamp lighting apparatus having a preheat function of preheating an electrode of the discharge lamp.
The discharge lamp lighting apparatus further includes an over resonance detection circuit which detects a high frequency current supplied to the discharge lamp load circuit and outputs a control signal to the switching element control circuit, and is constructed so that the switching element is controlled by the control signal from the protective circuit and the control signal from the over resonance detection circuit via the switching element control circuit. Therefore, it is possible to more precisely detect a fault, and to further improve a reliability of the protective circuit. Moreover, it is possible to apply the protective circuit to a discharge lamp lighting apparatus which is constructed in a manner that an oscillation frequency of the inverter circuit approaches a resonance frequency f0.
The discharge lamp lighting apparatus further includes a service interruption restoring circuit for automatically resetting the mask circuit when a feed from the direct current power supply is shut off, and after the feed is restored, the mask circuit is operated so as to mask a control signal outputted from the protective circuit to the switching element for a predetermined time. Therefore, even in the case where a service interruption takes place, after the service interruption is restored, it is possible to again operate the mask circuit, and to securely light the discharge lamp again simultaneously with when the power supply is restored.
Moreover, the present invention provides a discharge lamp lighting apparatus comprising: a direct current power supply; a switching element for switching a direct current supplied from the direct current power supply so as to generate a high frequency current; a discharge lamp load circuit which is constructed in a manner that a discharge lamp and a coupling capacitor are connected in series, and the discharge lamp is lit by a high frequency current generated by the switching element; a switching element control circuit for controlling the switching element. Further, the discharge lamp lighting apparatus includes a plurality of starting capacitors which are connected in parallel with the discharge lamp, at least one of the starting capacitors being connected to the switching element side with respect to the discharge lamp. Therefore, it is possible to make small an electrode loss consumed in the electrode of the discharge lamp, and thus, to improve an energy efficiency.
The plurality of discharge lamp load circuits having a coupling capacitor and a discharge lamp are driven by a high frequency current outputted from the switching element, and each of the plurality of discharge lamp load circuits is provided with a first voltage detecting unit for detecting a step-up voltage of each coupling capacitor, and converting the detected voltage into a direct current voltage; a second voltage detecting unit for detecting a drop voltage of each coupling capacitor, and converting the detected voltage into a direct current voltage; a first comparator unit for comparing the step-up direct current voltage detected by the first voltage detecting unit with a reference voltage; a second comparator unit for comparing the drop direct current voltage detected by the second voltage detecting unit with a reference voltage; and a control signal output unit for generating a control signal on the basis of an output from any of the first or second comparator units, and for outputting the single control signal to the switching element control circuit. Therefore, it is possible to detect a fault at the point of time when any of the discharge lamps is in a fault state, and to reduce the number of components as compared with the case where the comparator unit and the voltage detecting unit are independently provided in accordance with an increase of the discharge lamp load circuit.
The first voltage detecting unit includes a divided resistor and a constant voltage diode for dividing a voltage of each coupling capacitor, and a reverse current blocking diode interposed between the divided resistor and each coupling capacitor, and outputs the voltage divided by the divided resistor and the constant voltage diode to the first comparator unit, and the second voltage detecting unit includes a divided resistor and a constant voltage diode for dividing a predetermined voltage, and a reverse current blocking diode interposed between the divided resistor and each coupling capacitor, and outputs the voltage divided by the divided resistor and the constant voltage diode to the second comparator unit in the case where any voltage of each coupling capacitor is higher than the predetermined voltage, and further, is constructed in a manner that the predetermined voltage is applied to a coupling capacitor having a lower voltage via the reverse current blocking diode in the case where any voltage of each coupling capacitor is lower than the predetermined voltage. Therefore, even if the number of the discharge lamp load circuits is increased, the voltage detecting unit for detecting a voltage of each coupling capacitor is divided into the first voltage detecting unit for detecting a step-up voltage and the second voltage detecting unit for detecting a drop voltage, and thereby, it is possible to reduce the number of components of the voltage detecting unit by increasing the number of the divided resistors and the reverse current blocking diodes. Moreover, even if the number of the discharge lamp load circuits is increased, it is possible to detect the presence of the discharge lamp which is in the following states; more specifically, in a state such that any of the plurality of discharge lamps is in a fault state, that is, in a rectification lighting 1 state such that a detection voltage steps up as compared with the fully normal lighting state, in a rectification lighting 2 state such that a detection voltage drops as compared with the fully normal lighting state, and a detection voltage becomes 0 V by the removal of the discharge lamp. Moreover, it is possible to detect various faults of the discharge lamp.
In addition, the first voltage detecting unit outputs the voltage divided by the divided resistor and the constant voltage diode to the first comparator unit, and the second voltage detecting unit outputs the voltage divided by the divided resistor and the constant voltage diode to the second comparator unit in the case where any voltage of each coupling capacitor is higher than the predetermined voltage. Therefore, it is possible to largely set a difference in a reference voltage between the normal lighting state and the abnormal lighting state in the first and second comparator units, and thus, further improve a reliability of the protective circuit.
One end of the reverse current blocking diode of the second voltage detecting unit is connected to a starting capacitor side of the discharge lamp. Therefore, when the number of the discharge lamp load circuits is increased, in the case where any of the discharge lamps is dismounted, a circuit of the coupling capacitor of the discharge lamp and the reverse current blocking diode is shut off; as a result, the discharge lamp all becomes a normal state in the second voltage detecting unit, and the presence of the discharge lamp is not detected, and thereby, it is possible to make a detection in only case where any of the plural discharge lamps is in an abnormal state and a normal state.
The first voltage detecting unit includes a divided resistor and a constant voltage diode for dividing a voltage of each coupling capacitor, and a reverse current blocking diode interposed between each constant voltage diode and the first comparator unit, and outputs the voltage divided by the divided resistor and the constant voltage to the first comparator unit via the diode reverse current blocking diode, and the second voltage detecting unit includes a divided resistor and a constant voltage diode for dividing a predetermined voltage, and a reverse current blocking diode interposed between the constant voltage diode and each constant voltage diode of the first voltage detecting unit, and further, a direct current voltage of each coupling capacitor is dived by a divided circuit comprising the divided resistor and the constant voltage diode, and the divided voltage is inputted to the first comparator unit via each reverse current blocking diode, and a direct current voltage of a direct current power supply is dived by a divided circuit comprising the divided resistor and the constant voltage diode, and the divided voltage is inputted to each coupling capacitor via each reverse current blocking diode. Therefore, it is possible to use a reverse current blocking diode having a low withstand voltage.